French Patent No. 90 13,049 mentions using fluorescent plastic optical fibers for detecting the durations of arcing in circuit-breakers using gas, in particular sulfur hexafluoride (SF.sub.6).
By observing arcing durations, changes in the state of the apparatus can be assessed, and maintenance work can be planned.
Vacuum circuit-breakers require near-perfect gastightness. They are made gastight once and for all in the factory. It is therefore impossible to re-evacuate them at a later stage. Their casings are often made of ceramic, and are therefore opaque, thus making it difficult to inspect the state of the circuit-breaker visually.
A known method of verifying the internal state of the circuit-breaker is to measure its electron emission or ionization threshold, sometimes via an external magnetic field.
In practice, such inspection is performed during a scheduled period of maintenance.
The state of the apparatus is therefore not monitored continuously.
It is well known that, in the presence of a sufficient voltage, a vacuum circuit-breaker emits X-rays while it is being closed, or while it is being opened.
To this end, reference is made to the article entitled "Limiting X-radiation from a high voltage vacuum interrupter at the prebreakdown stage", by W. GORCZEWSKI, report 34-01, 7th International Symposium on High Voltage Engineering, DRESDEN, August 1991.
The X-rays emitted while a vacuum "bottle" is opening or closing are relatively low-level. Protective devices are provided to render them harmless.
It is also known to use scintillation optical fibers for detecting radiation from particles in high-energy accelerators.
For example, reference is made to the article "Organic Scintillators with large Stokes shifts", The Journal of Physical Chemistry, vol. 82, No. 4, 1978, or to the article "Plastic fibers in high energy physics" by M. Blumenfeld, NIM, 257 (1987).
Scintillation fibers pick up radiation via their peripheries and transform it into a light wave passing along the fiber.
The level of X-radiation depends essentially on the voltage applied, the distance between the contacts, the material, the state of the contacts, and the state of the vacuum. Loss of vacuum considerably reduces X-radiation. Therefore, the absence of X-rays, after the circuit-breaker has been operating for a certain length of time, may indicate a loss of vacuum. In the same way, a change in the spectrum of the radiation recorded may signify that there has been an operating change in the inter-electrode space, e.g. the contacts have been eroded.
As explained below, it was the Applicant who thought of using scintillation fibers to obtain continuous and effective monitoring of a circuit-breaker including vacuum bottles by measuring the vacuum inside the bottles.
This use makes it possible to solve the specific problems that arise, namely the following:
the measuring means used must withstand the voltage existing between the terminals of the vacuum bottles (in the range 6 kV to 72 kV);
it must be possible to measure the vacuum without taking the circuit-breaker apart;
the measuring means used must be compact, so as to be housed as close as possible to the vacuum bottle, because the X-radiation from a vacuum bottle is low-level;
the response time must be short; and
the measuring means must be cheap, and in particular must make it possible to use cheap components, such as photo-diodes.
These objects cannot be achieved by the apparatus described in Document EP-A-0,309,852 which advocates using a Geiger-Moller counter.